The invention relates to wireless communications, more particularly to wireless local area networks, and still more particularly to the operation of wireless local area networks in the presence of periodic interference.
Many portions of the radiofrequency spectrum are designated unlicensed bands, and these bands are presently widely used by wireless local area network (WLAN) technology. Because they are unlicensed, these bands are free for use by any device, so interference avoidance and/or tolerance are considerations in the design of such technology.
WLAN technology that complies with IEEE 802.11 standards is known as “Wi-Fi” technology. A Wi-Fi deployment involves a so-called access point (AP) to which some number of so-called “stations” (STA) can wirelessly connect. Wi-Fi technology operates in an unlicensed band, and the sharing of this spectrum is achieved by dividing the total bandwidth into a number of channels. The channels are typically 20 MHz wide in the 2.4 GHz band, where up to 13 channels are defined. These channels partially overlap one another, and thus will interfere with one another. Typically, three non-overlapping channels are used in the 2.4 GHz band. For the 5 GHz band, which has a much larger available bandwidth, many more channels are available. However, with the development of the IEEE 802.11n and IEEE 802.11ac standards, the bandwidth of the channels has been increased from 20 MHz to 40, 80, and even 160 MHz. Thus, the number of non-overlapping channels is still rather small, particularly when the wider bandwidths are used.
In typical WLAN deployments involving more than one AP, channels are allocated in a manner that attempts to avoid having one AP's channel overlap those of another. In practice, this often means that one tries to maximize the distance between APs using the same channel, so that the transmissions of one will not interfere with the transmissions of another. The strategy adopted for channel access is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), in which the channel is first sensed to detect its possible use by one or more other transmitters. Transmission is initiated only if the sensing results in the channel being declared Idle. If the channel is instead declared Busy, the transmission is essentially deferred until the channel is found Idle.
When the range of several APs using the same frequency is sufficiently large, the coverage areas of the APs will overlap. Combined with the use of CSMA/CA, this means that all transmissions related to one AP will be deferred whenever a transmission on the same frequency to or from another AP that is within range is detected. Effectively, this means that if several APs are within range, they will have to share the channel in time, and the throughput for the individual APs may be severely degraded.
Cellular communication networks also employ wireless technology, and these have historically utilized licensed rather than unlicensed bands. However, discussions have recently started relating to the use of the unlicensed bands by one such network, the so-called Long Term Evolution (LTE) network that is standardized by the Third Generation Partnership Project (3GPP). One approach involves the LTE network operating on a best effort basis by using the unlicensed band as carrier aggregation of a secondary cell (SCell) while operating a Primary cell (PCell) in licensed spectrum. Thus, the network's connection to the device will still be maintained via at least the PCell even if interference by, for example, Wi-Fi or other technology (e.g., Bluetooth®) prevents communication on the SCell.
The present discussions are focusing on the SCell being used only for down-link (DL) communications, thus requiring the use of a PCell for up-link (UL) communications. However, it can be envisioned that, in the future, there may be LTE (or other) systems operating in the ISM band without the need for a simultaneous link in a licensed band. Such a system would most likely use time division duplex (TDD) to achieve shared usage of channels. Additionally, one of the more important goals in the evolvement of the IEEE 802.11 standard is to increase the spectrum efficiency in very dense deployments. This development work is done within the IEEE 802.11ax task group (TG). As one option for the 802.11ax standard, the use of a more scheduled approach has been discussed; that is, one in which channel access is controlled by the AP, rather than distributed. Thus, it is possible that there may be future 802.11-compliant technology whose operation is based on TDD.
In order to ease the description and avoid the use of the cumbersome generic terms “WLAN-type” and “LTE-type”, as used herein the term “WLAN” is used generically to cover all types of ad hoc local area networking technology, such as and without limitation, Wi-Fi (i.e., IEEE 802.11 compatible) technology and Bluetooth® (i.e., Bluetooth SIG-defined) technology. Similarly, references to LTE should be construed broadly to cover any type of other network having relevant characteristics similar to the proposed LTE systems described herein (e.g., the use of the unlicensed band without conforming to WLAN standards for ad hoc local area networking, and transmissions having a predictable periodic nature).
The inventor has recognized that the situation involving concurrent use of the unlicensed band by different types of systems (e.g., concurrently by WLAN and LTE systems) can cause problems for the WLAN system. For example, if another system, (e.g. an LTE system as defined by 3GPP standards) is used in the same band as the WLAN network, this use may severely degrade performance within the WLAN network if no counteraction is taken. In particular, because of the CSMA/CA mechanism, the WLAN system may defer a transmission upon sensing that the spectrum is Busy, and this may happen even if the signal quality (e.g., the signal-to-interference-ratio (SIR)) for performing the transmission would have been sufficient to allow that transmission to be successful. In addition, there is a high likelihood that an ongoing WLAN transmission will experience interference from an LTE transmission because the channel access in WLAN is typically based on distributed access, (e.g., using the distributed coordination function (DCF), which is a mechanism whereby each potential user of the spectrum that is presently sensed as “Busy” applies an algorithm to decide how long to defer its next transmission attempt).
In scenarios in which an LTE (TDD) node operates on the same or adjacent channel to one used by a WLAN deployment, the LTE transmission may cause severe interference for the WLAN system. The LTE interference may cause problems for both the AP (i.e., for the UL transmission) and the STA (i.e., for the DL transmission). The severity of interference that any AP or STA will experience depends, in part, on the distance from the LTE transmitter and in part on the output power used by that transmitter. It is noted that any link in LTE (i.e., either DL or UL) may interfere with any of the WLAN links (either DL or UL). Accordingly, as used in this discussion, “LTE transmitter” may refer either to the network node (e.g., eNB) or to a user equipment (UE). As discussed initially, there may be a situation in which only DL transmissions are performed in the unlicensed band as a secondary carrier, whereas all UL transmissions take place in a licensed band. In this case, it is of course only the LTE DL transmission (i.e., transmissions from the eNB) that can cause interference that disturbs the WLAN links.
The power level of the received interfering signal relative to the power level of the received desired signal (i.e., the signal from the AP or one of the stations) will determine whether the interference level will be high enough to cause an issue. Another factor that affects whether the WLAN deployment will experience interference is the level of activity of the interfering transmitter. If, for example, it is an eNB that is interfering, it can be expected that the amount of activity will be very high because the eNB transmits downlink data to many UEs within its serving area. On the other hand, if it is only one UE that is causing the interference and the UE has very low activity, the interference may only be present for a small fraction of the total time.
It is therefore desired to have technology that can mitigate or avoid the potential degradation of WLAN performance that can result from periodic interference such as, but not limited to, interference from an LTE TDD system operating in the same band as the WLAN deployment.